1. Field of the Invention
This invention relates to viewporting and scrolling in a display having an all points addressable graphic display capability.
2. Prior Art
It has been known to provide a multiple data window display for displaying data from independent application programs in a multi-tasking environment on a common display screen. In one such prior system, plural screen buffers are provided for storing scan image defining data, and control means have been provided for selectively coupling the output of a given single one of the plural screen buffers to video means at any given time so that at any given point on the display screen, the data displayed originates from a selected one of the screen buffers. If, in such a system, the screen buffers operate synchronously and can be accessed on the same bases, for example, on a byte basis, a composite screen picture can be assembled from segments of the selected screen buffers without overlaps or gaps in the resulting mosaic picture. Systems of the foregoing kind are shown and described in U.S. patent application Ser. Nos. 542,572 and 542,376, filed Oct. 17, 1983.
A problem exists however where the buffers operate under the control of systems in which data is accessed differently, for example, on 9-bit byte boundaries from one system and 16-bit half word boundaries from another system. A problem of this kind can arise where the screen is notionally divided into "character boxes" 9 bits wide for accommodating alphanumeric characters designed for a 9-bit wide format, but where pel data for all points addressable (APA) graphic presentation is derived from a different system having a pel buffer organized around 8-bit bytes and 16-bit half words, so that congruency between the two systems can be achieved only on boundaries which are divisible both by 9 and 16. The smallest string of pels which could be brought into exact registration from two buffers in such a system would be nine 16-bit half word and sixteen 9-bit bytes. Thus if an effort were made to fit pel data from 16-bit half words into a viewport on the screen defined by 9-bit wide character boxes or spaces, only rarely would the pel data exactly fit. Therefore, if the whole of the APA window data is to be shown within the screen viewport, there arise occasions when a portion of the viewport is unfilled by the smaller window of data, leaving a gap on the screen, distracting from the appearance of the display.
Another problem arising from mismatch of system parameters, arises when scrolling on a pel basis is desired. Scrolling is frequently useful where the screen viewport is smaller than the body of data to be shown, so that the user desires to, in effect, move the viewport across the body of data so that the data appears to scroll within the viewport. If the viewport and the data are organized on different radices as above described, the scrolling system must take account of both in the fetching of data to be shown and the moving of it into changeable, or constantly changing, relationships to the viewport.
A third problem arising in systems as above described is in adjusting the overall timing of both systems so that input/output operations with respect to the pel buffer can be facilitated and yet the desired image registration maintained.